Communications devices, such as mobile phones and the like, use filters and sub-systems incorporating filters (such as duplexers, diplexers, and the like) to separate signals in different frequency bands, such as transmission and reception signals, for example. There has been a trend for such devices to be miniaturized and to have a reduced overall part count. Furthermore, it is desirable for the attenuation characteristics of the filters, and isolation between different filters, to be improved.
FIG. 1 is a schematic diagram illustrating a conventional filter device, such as is disclosed in U.S. PG-Pub No. 2013/0113576, for example. The filter device 100 includes a first filter 110 having a first passband and connected between a common terminal 101 and a transmission terminal 102, and a second filter 120 having a second passband and connected between the common terminal 101 and a reception terminal 103. This type of filter device can be used as a duplexer, for example. The filter device 100 further includes a longitudinal-coupling-type acoustic wave resonator or acoustic wave delay line 130 including three Interdigital Transducer (IDT) electrodes and connected between the transmission terminal 102 and the reception terminal 103. In addition, a first capacitor 105 can be interposed between the transmission terminal 102 and the longitudinal-coupling-type acoustic wave resonator or acoustic wave delay line 130, and a second capacitor 106 can be interposed between the reception terminal 103 and the longitudinal-coupling-type acoustic wave resonator or acoustic wave delay line 130. In this filter device 100, the longitudinal-coupling-type acoustic wave resonator or acoustic wave delay line 130 can provide a cancelling circuit for cancelling out signal leakage from the transmission terminal 102 to the reception terminal 103 to improve the isolation characteristics of the filter device 100.
When a temperature-compensation-type surface acoustic wave element (TC-SAW) is used for the resonator(s) forming the first filter 110 and/or the second filter 120 in the conventional filter device as shown in FIG. 1, a higher-order mode may occur in a frequency band that is 1.2 to 1.3 times greater than the center frequency of a main mode of the relevant filter. Because this frequency band can be used for communication via carrier aggregation (CA), it is desirable to suppress the higher-order mode, and therefore the attenuation characteristics of the filter device 100 need to be further improved to achieve the desired suppression.